Operating Systems Lecture 3: Process Concept

Study Notes

Operating systems execute various programs as processes, which are programs in execution.
A process must progress sequentially; no parallel execution of instructions occurs within a single process.
Key components of a process include:
- Program Code (Text Section)
- Current Activity: Includes program counter and processor registers.
- Stack: Stores temporary data such as function parameters, return addresses, and local variables.
- Data Section: Contains global variables.
- Heap: Memory allocated during runtime dynamically.

A program remains passive when stored on disk (executable file); it becomes an active process when loaded into memory.
Program execution can start from GUI mouse clicks or command-line entries.
One program can have multiple processes, allowing for concurrent users accessing the same application.

PCB contains vital information about each process, also known as the task control block.
Key elements within PCB include:
- Process State: Indicates current status (e.g., running, waiting).
- Program Counter: Points to the next instruction to execute.
- CPU Registers: Holds values of all process-centric registers.
- CPU Scheduling Information: Contains priorities and scheduling queue pointers.
- Memory-Management Information: Details memory allocated to the process.
- Accounting Information: Tracks CPU time used, elapsed clock time since process start, and time limits.
- I/O Status Information: Lists I/O devices allocated to the process and open files.

Initially, processes have a single thread of execution.
Multiple threads of control can be created, requiring storage for additional program counters in the PCB.

The process scheduler is responsible for selecting the next process for CPU execution.
Goals include maximizing CPU usage and maintaining scheduling queues:
- Ready Queue: Includes processes in main memory that are ready and waiting for execution.
- Wait Queues: Includes processes waiting for I/O or other events.
Processes transition between various queues as they execute.

Commonly visualized through queueing diagrams illustrating the flow of jobs between the ready queue, CPU, and I/O queues.
Illustrates how processes interact with CPU and handle I/O requests.

Occurs when the CPU transitions from executing one process to another.
Involves saving the current process state into its PCB and loading the state of the next process.
Context switches can occur due to interrupts or system calls.